An obvious plastic/rubber sword is fine. We brought them onboard with no problem.

From the DCL website:

Question
What items are not permitted on board?Answer
In order to maintain a safe and secure environment for our Guests and crew, the following are examples of items that Guests are not allowed to bring on board. These and other similar items will be confiscated upon being found.
"Weapons and firearms (including ammunition, knivesincluding dive knivesswords, daggers or any other types of weapons) are not allowed onboard under any circumstances
"Realistic replicas of firearms and weapons